April 17, 2007

New research sheds light

New research from a group of University of Chicago and French scientists lends new meaning to the phrase “the sun’s rays beating down.” The researchers managed to move a stream of liquid using only the force of light.

The team shined a laser onto a soap-like liquid, causing the liquid to flow along the path of the beam for a surprisingly long distance before breaking up and recirculating. Although lasers have been used before to move single particles, the researchers believe this is the first time light has been used to generate bulk flow in a liquid.

Physicists have known for some time that lasers can move liquids by heating them, but that isn’t what happened in this case. Instead, the force of the light itself deformed the surface of the soapy liquid, a mixture of oil and water specially designed for the experiment.

When the photons—particles of light—collided with the liquid, they scattered. The photons didn’t transfer any energy to the liquid in the collisions, so there was no heating. However, “this scattering transfers momentum to the liquid, which produces a flow,” said Robert Schroll, a graduate student in physics at the University and lead author of the report, in an e-mail interview.

All light exerts a small push on the objects it strikes. This is known as “radiation pressure.” Usually this goes unnoticed, but in this case the liquid’s surface was so weak and the photons so concentrated that there was an observable effect.

The team discovered the phenomenon while Wendy Zhang, assistant professor in physics at the U of C, was visiting colleagues Regis Wunenberger, Alexis Casner, and Jean-Pierre Delville at the University of Bordeaux I. Zhang went to Delville’s lab, where he had previously shined a less intense laser beam on the same soapy liquid.

To see what would happen, Delville turned up the power. The liquid formed a jet narrower than a human hair almost a millimeter in length, much longer than the scientists were expecting.

The researchers hope to have some more impressive demonstrations in the future. “We were just trying to show evidence of this flow, not trying to do anything fancy,” Schroll said. Using the current laser system, Schroll thinks they could create a jet almost two centimeters in length before the beam becomes too scattered.

Physicists may also be able to create more than single streams. “Our hope is that by using several laser beams, one could direction fluid in more complicated manners,” Schroll said.

The technology could have applications for microfluidics, the science of controlling fluids on very small scales, such as those on tiny chips in laboratory equipment. Mechanical pumps are currently used to drive such fluid through small channels etched on a chip. “Having the flows driven through the bulk fluid by lasers could simplify the fabrication and use of such devices,” Schroll said.